Improved insulation systems for exterior walls of residential and commercial structures have received interest in an age of rising energy costs, and concerns about greenhouse gas emissions of heating fuels.
Standard frame walls are 8 feet to 12 feet high. Older homes and warmer climates typically utilize nominal 2×4 in. wall studs (having actual cross-section dimensions of 1.5 inches by 3.5 inches). Newer homes in colder climates may utilize nominal 2×6 studs (actually 1.5 inches by 5.5 inches), to allow more space in the wall cavity for insulation.
Framing consists of top and bottom plates, wall studs and headers, with trimmers and king studs as needed for window and door openings. Exterior sheathing, typically 4×8 (or 4×9) oriented strand board or plywood sheets, is nailed to the outer face of the frame formed by the wall studs. Interior wallboards, typically gypsum-containing wallboards, are then fastened to the interior facings of the studs, after insulating, wiring, and plumbing of the insulation cavity is completed. The depth of the insulation cavity depends on the depth of the studs used, with a cavity 3.5 inches deep for 2×4 in. studs, and 5.5 inches deep for 2×6 in. studs.
A long-known form of insulation comes in the form of fibrous insulation, including mineral fiber, glass fiber, cellulose, cotton, polyester fiber, wool, and wood fiber. One common embodiment is fiberglass batts that are unrolled and cut (if necessary) to fit in the space between support studs of a building, generally occupying the entire depth of the insulation cavity between the exterior sheathing and interior walls.
A more recent advance has been the use of cellular plastic insulation in place of fibrous insulation. Cellular plastic insulation includes products such as spray-applied polyurethane, polyurethane foam board, polyisocyanurate, Icynene®, phenolic foam, and polystyrene foam and board stock. Polyurethane foam is a common cellular plastic insulation, which may be open cell or closed cell, and is applied into the cavities between the studs, starting at the inner facing of the exterior sheathing, and applied from the inner facing to a predetermined thickness. Closed-cell cellular plastic insulation has advantages over fibrous insulation, including superior vapor retarding properties and superior thermal resistance per unit thickness. Cellular plastic insulation has a disadvantage of being higher in cost than fibrous insulation products.
There are a number of terms used to quantify the thermal performance of an insulation product. The most common is the thermal resistance, or R-value, which is the ratio of the temperature difference across an insulator to the heat flux (heat flow per unit area) through the insulator. In the United States, R-value is usually expressed as a quantity in ft2·° F.·h/Btu. All R-values in the application are presented in these units. The larger the R-value, the better the building insulation's effectiveness. The R-value can be increased by increasing the thickness of the applied layer. Closed-cell foam has the advantage over fiberglass insulation of providing higher R-values for the same thickness of insulation. However, because cellular plastic insulation is an expensive building component, a need exists for a system to enhance the thermal performance of an insulation system by a means other than simply increasing the amount of insulation used in the cavity.